ABSTRACT
ABSTRACT: The highly variable nature of the deeply weathered Oporto granite posed significant challenges in the driving of the 2.3-km-long C line and the 4-km-long S line of the Oporto Metro project. Two 8.7-m diameter Herrenknecht EPB TBMs were used to excavate these tunnels and early problems were encountered due to over excavation and face collapse. Three collapses to surface occurred in the first 400 m of the C line drive and the last of these collapses resulted in the death of an occupant of a house on the surface. This resulted in a delay of almost 9 months to the project and the difficulty was finally resolved by the introduction of an Active Support System which involves the injection of pressurized bentonite slurry to compensate for deficiencies in the face support pressure when driving in mixed face conditions. Both the C and S lines have now been completed with minimal surface subsidence and no face instability.
